The subject invention relates to a magneto-resistive CrO2 polymer composite blend for use in magnetic storage devices, such as in audio and video tapes, magnetic read heads, magnetic field probes, or current voltage sensors in electrical devices and the process for preparation of the same to provide a matrix for conducting and magnetic fillers to form a blend which in turn shows the desired magneto-resistive property.
The present invention also resides in the process for the preparation of a magneto-resistive polymer composite blend comprising a polymer, preferably Low Density Poly Ethylene (LDPE), the ferromagnetic filler preferably Chromium Oxide capable of exhibiting magnetic properties and an additive preferably conducting carbon brings about magneto-resistive properties in the composite.
The embodiment of the invention resides in the fabrication of the conducting polymer blend which would mimic the properties of ferromagnetism and metallic conductivity of the bulk CrO2 and to characterize its electrical property in the presence of an external magnetic field.
Materials exhibiting magnetoresistance ratio greater than a few percent are useful in a variety of devices. The devices utilize the magnetoresistive materials"" ability to respond, by way of resistive changes, to small changes in applied magnetic field. This effect is useful, for example, in magnetic sensing devices, current sensing devices, memory elements, or even acoustic devices. Examples of useful devices are discussed, for example, U.S. Pat Nos. 5,450,372 and 5,461,308.
K. Chanhara et al., Applied Physics Letters, Vol 63 (14), at 1990; R Von Helmholt et al., Physical Review Letters Vol. 71 (14) at 2331; and U.S. Pat. Nos. 5,549,977 and 5,538,800 teach that desirable MR has been observed in mixed metal oxides, e.g. Laxe2x80x94Caxe2x80x94Mnxe2x80x94O, Laxe2x80x94Baxe2x80x94Mnxe2x80x94O and Laxe2x80x94Srxe2x80x94Mnxe2x80x94O. The magneto resistance of Laxe2x80x94Srxe2x80x94Mnxe2x80x94O perovskites, appears to be better in polycrystalline samples, as opposed to single crystals, possibly due to spin-polarized tunneling of electrons between grains.
H. Y. Hwang et al., Physical Review Letters, Vol. 77 (10), at 2041 teaches in particular, it has been found that trilayer structures using Laxe2x80x94Caxe2x80x94Mnxe2x80x94O and Laxe2x80x94Srxe2x80x94Mnxe2x80x94O perovskites undergo a change in resistance by a factor of 2 in a low applied field of 200 Oe, indicating the potential use of such materials in field sensors. Se J. Z. Sun et al. Appl. Phys. Lett., Vol. 69 (21). 3266. In (U.S. Pat. No. 5,856,008), Cheong et al teaches that in a bulk polycrystalline CrO2 material shows low temperature magneto-resistance ratio of 6% at 5 K at 20 kOe. The materials is synthesized by carefully incorporating insulating grains of Cr2O3 between ferromagnetic clusters of CrO2 grains. By introducing conducting barrier between grains, the MR ratio is increased. Electron can hop between grains provided the moment of each grain is aligned parallel to each other. However it is noted that the grains are usually mis-aligned, as a result the resistance at low temperatures. Other examples of suitable devices that rely upon a magnetoresistive material are discussed, for example, U.S. Pat. Nos. 5,450,372 and 5,461,308, referenced above, as well as U.S. Pat. Nos. 5,422,571; 5,565,695, 5,432,373, 5,541,868.
A new property such as magneto-resistance was realized in CrO2 recently. This property exhibits a large drop in resistance on application of an external field. The sensitivity with which it can change its resistance in the presence of an applied magnetic field determines whether this compound could be used as magnetic sensors.
Such magneto-resistive features have been exemplified in a variety of compounds which includes ferromagnetic Fe and Co based alloys, rare earth manganites. While, alloys exhibit this property in thin film configuration when stacked as magnetic and non-magnetic layers, manganites show such drop in resistance both in bulk and as thin films. This property if realized as a polymer, it can be exploited in a variety of applications due to ease of processing such a compound to any desired size and shape. This challenge therefore lies on the design of a polymer blend which can show such unique magneto-resistive behavior.
Electrically conducting polymer composite materials exhibiting positive temperature coefficient of resistance effect have been in use in resistance switching devices for many years. These materials are characterized by a switch temperature at which the material resistivity changes by orders of magnitude. The most studied polymer composite system which exhibits this effect consists of polyethylene loaded with carbon black. At temperatures below 130xc2x0 C., i.e. below the melting point of polyethylene there is an anomalous resistance which raises by orders of magnitude. This increase in resistance is believed to be due to the increased carbon black particle separation which forms a discontinuous polyethylene phase expansion. Upon meting, conducting polymer composite materials have been realized in co-polymer blends too, such as polyethylene-polystyrene copolymers wherein conducting fillers such as carbon black can bring about electrical percolation at doping levels below 3 wt %. As a result, a co-polymer which otherwise shows a resistance of the order of MOhms, shows below 1000 Ohms when doped with conducting filler such as carbon black. This aspect in particular is used in sensor technology.
Conducting polymer composite materials consisting of a random distribution of a conducting filler throughout an insulating polymer are of interest for several applications. CrO2 being an important electronic material finds application as a recording material owing to its unique property of being ferromagnetic and metallic at room temperatures. Studies on CrO2 are limited owing to its thermal instability. The compound decomposes above 300xc2x0 C. which limits its application. The key tools in the design of a CrO2 based polymer blend are in appropriate polymer and selective localization of CrO2 grains in the blend. The best way to fabricate such a composite is to selectively disperse the CrO2 conducting chain with the filler in the insulating polymer matrix so that it forms a current conducting chain with the filler concentration as low as possible. An essential prerequisite for technological application is to retain the aciculate nature of the CrO2 grains during composite processing.
The object of the present invention is to improve the conductivity between the CrO2 grains embedded in a highly insulating polymer phase by adding an additive. The proportion of this additive in the polymer:CrO2:additive ratio varies from 3-10 wt %. Percolation between the CrO2 grains is achieved even with 3% of this additive.
However a nominal composition of  greater than 3% is always preferred to keep the material below 100 Ohms. As a result of effective percolation between the magnetic CrO2 particles through a conducting additive network the response of the CrO2-polymer blend can be measured by applying an external magnetic field.
Chromium (iv) oxide, CrO2 is known for its unique application as a tape material. It is a metal and at room temperature Ferro-magnet having a Curie Temperature of 385xc2x0 K and at T=0K, it""s a half metallic Ferro-magnet. This oxide is hydro-thermally prepared and has fine grained needles with acicular shape, a feature desired for magnetic storage applications.
Accordingly the present invention relates to a magneto-resistive CrO2 polymer composite blend that can be made into a film or artefact for use in magnetic storage devices such as audio and video tapes, magnetic read heads, magnetic field probes or current voltage sensors in electrical devices, comprising:
88%-93% w/w of low density polyethylene;
5-8% w/w of CrO2; and
2-4% w/w carbon black.
The said polymer, preferably low density polyethylene is having a mesh size 200 micron and 99.9% purity and the said magnetic filler is in the form of powder.
The polymer composite blend is having the melting temperature of about 95xc2x0 C.
The invention also resides in the process for the preparation of a magneto-resistive CrO2 polymer composite blend, comprising
mixing the said polymer, said CrO2 and said additive as conducting carbon in a mortar pestle to obtain a homogenous mixture
heating the said homogenous mixture at a temperature of 95-100xc2x0 C. to obtain a blended melt
casting the said melt between two metal plates to obtain sheets by applying uniaxial pressure.
The pressure applied to obtain the said sheet is in the range of from 4-5.5 KPa.
Alternatively in the process for the preparation of a magneto-resistive polymer composite blend, the said polymer, said CrO2 and said additive were mixed to obtain a homogenous mixture, the said homogenous mixture is transferred into a die to obtain a pellet by applying a pressure of 4.5-5.5 KPa. The pellet so obtained is placed in a metallic mould and kept in a preheated hot press machine to allow the said polymer to melt at a pressure of 4-5.5 KPa and flow into thick sheet . The said mould is then cooled to obtain a film having magnetic properties.
The said mould used in the subject invention is preferably made up of aluminum and said hot press machine is uni-axial hot press machine
The size of the said films so produced varies from 10 mm to 25 mm in diameter depending on the size of the powder compact of the mixture.
The subject application may better be understood with reference to accompanying drawings. However, the same should not be construed to restrict the scope of the application as they are for illustrative purposes only.